Passing values in Python

python pass-by-reference pass-by-value
Joan Venge picture Joan Venge · Feb 10, 2009 · Viewed 82k times · Source

When you pass a collection like list, array to another function in python, does it make a copy of it, or is it just a pointer?

Answer

Stephen picture Stephen · Feb 10, 2009

Python passes references-to-objects by value.

Python passes references-to-objects by value (like Java), and everything in Python is an object. This sounds simple, but then you will notice that some data types seem to exhibit pass-by-value characteristics, while others seem to act like pass-by-reference... what's the deal?

It is important to understand mutable and immutable objects. Some objects, like strings, tuples, and numbers, are immutable. Altering them inside a function/method will create a new instance and the original instance outside the function/method is not changed. Other objects, like lists and dictionaries are mutable, which means you can change the object in-place. Therefore, altering an object inside a function/method will also change the original object outside.

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